The present invention relates to a stable power supply apparatus connected to a power system, and more particularly to a stable power supply apparatus utilizing a secondary battery, such as a redox flow battery, a sodium sulfur battery or a lead-acid battery.
In recent years, electronic apparatuses which are sensitive to fluctuations in a power supply voltage, such as personal computers and precision electronic apparatuses equipped with precision motors, have been used frequently. Hence, the need for countermeasures against an instantaneous voltage drop (hereafter simply referred to as an instantaneous drop), a phenomenon wherein the power supply voltage lowers significantly for a short time, is growing, and demand for stable power supply apparatuses serving as instantaneous voltage drop countermeasure apparatuses is increasing. In addition, it is required to add an instantaneous voltage drop countermeasure function to various kinds of stable power supply apparatuses having been used conventionally, such as a load leveling apparatus, a peak cut apparatus, a frequency fluctuation suppressive apparatus, a voltage regulating apparatus and a flicker countermeasure apparatus. The state wherein the above-mentioned load leveling apparatus, peak cut apparatus, etc. are operating in accordance with their ordinary functions, such as load leveling and peak cutting, is hereafter referred to as “normal time.”
FIG. 11 is a block diagram of a power supply system having a conventional stable power supply apparatus 101 using a redox flow battery as a secondary battery. In the figure, a system bus line 102 is connected to the power system 100 of a substation 130 via a transformer 120. The stable power supply apparatus 101 is connected to this system bus line 102 via a switch 103. To the system bus line 102, important loads 104 and 105, for example, are connected via switches 114 and 115, respectively. The important loads 104 and 105 are, for example, important facilities of bulk power customers requiring particularly stable power supply, such as semiconductor production plants and precision machining plants. A general load 110 is also connected to the system bus line 102 via switches 109 and 111. The stable power supply apparatus 101 comprises a transformer 106 mainly serving as an interconnecting reactor, a converter 107 for converting AC power to DC power or vice versa, and a redox flow battery serving as a large-capacity secondary battery 108. This stable power supply apparatus 101 functions as a peak cut apparatus or a load leveling apparatus in normal time; however, it also operates as an instantaneous voltage drop countermeasure apparatus for preventing operation stop or the like of the important loads 104 and 105 during an instantaneous drop owing to the occurrence of a lightning accident or the like.
The function of the stable power supply apparatus 101 will be described below in detail. In “stationary time,” a state wherein the supply and demand state of the power in the power supply system connecting the substation 130 to the important loads 104, 105, and the general load 110 is balanced, the AC power supplied from the system bus line 102 to the stable power supply apparatus 101 via the switch 103 and the transformer 106 is converted into DC power by the converter 107 and charges the secondary battery 108. On the other hand, in the case that the power consumption of the important load 104 or 105 increases significantly and the power supplied to the loads 104 and 105 transiently exceeds the capacity of the substation 130, this state is detected by a detection circuit 8 having a voltage detector 10 and a current detector 11. The detected output of the detection circuit 8 is fed to a control circuit 9. The control circuit 9 controls the converter 107 to convert the DC power discharged from the secondary battery 108 into AC power with the converter 107 and supplies an active power corresponding to the amount of the above-mentioned excess to the system bus line 102, thereby stabilizing the supply and demand. The power corresponding to the excess amount that should be supplied from the system bus line 102 is supplied from the stable power supply apparatus 101 instead, whereby the peak of power supplied from the system bus line 102 can be cut; hence, this function is referred to as “peak cut.”
In the case that the capacity of the secondary battery 108 of the stable power supply apparatus 101 is made larger so that a power capable of being supplied for a long time can be stored, the apparatus can be used as a load leveling apparatus. In other words, the secondary battery 108 is charged with constant power for a constant time (about eight hours in a typical case) during a low consumption time zone at night, and power is supplied from the secondary battery 108 at constant power for a constant time (about eight hours in a typical case) during a high consumption time zone in the daytime. Hence, a power exceeding the servable power of the substation 130 can be supplied during the high consumption time zone. The stable power supply apparatus for this use levels the large difference between the power demand in the daytime and that at night; hence, the apparatus is referred to as a “load leveling apparatus.”
In the case when the power system 100 was struck by lightning and an instantaneous drop occurred in the voltage of the system, the voltage detector 10 detects the instantaneous drop. The voltage of the system must be restored immediately to the voltage before the instantaneous drop in order to prevent operation stop and the like of the important loads 104 and 105 due to the instantaneous drop. For this purpose, the stable power supply apparatus 101 controls the converter 107 with the control circuit 9 and supplies reactive power and active power from the secondary battery 108 to the important loads 104 and 105 via the converter 107 and the system bus line 102 so as to maintain the stable supply of power. When the power supplied from the secondary battery 108 is insufficient, the switch 109 is opened to disconnect the general load 110 that is low in importance so as to supply desired power to at least the important loads 104 and 105 and to prevent operation stop of the important loads 104 and 105. Immediately after recovery from the instantaneous drop, the converter 107 is recovered to its normal operation state, and power is supplied.
In the conventional stable power supply apparatus, during peak cut time and load leveling time, it is necessary to supply, for example, a power of about 500 kW in the former case or a power of about 2 MW in the latter case for a relatively long time (for example, about one hour in the former case or about eight hours in the latter case). When an instantaneous drop occurs owing to lightning during peak cut or load leveling operation, it is necessary to additionally supply a power of one to several MW for a relatively short time (for example, two seconds) in order to recover the lowered voltage.
Secondary batteries, such as a redox flow battery, a sodium sulfur battery and a lead-acid battery, have “instantaneous large-current supplying capability”, whereby they can supply, for several seconds to several minutes, a current about several times the rated current thereof that can be supplied in normal time. By utilizing this capability at the time of an instantaneous drop, countermeasures against the instantaneous voltage drop can be taken without increasing the rated capacity of the secondary battery. For this reason, the conventional stable power supply apparatus was equipped with the large converter 107 having a large power capacity corresponding to the instantaneous large current supplying capability of the secondary battery. It was necessary to set the power capacity of the converter 107 to several times the power capacity required in normal time, for example.
Such an instantaneous drop owing to lightning does not occur very frequently, about 20 times at most in a year. In addition, the duration of an instantaneous drop owing to lightning is several seconds even in the case of multiple lightning. A short circuit or a ground fault may occur in a rare case when a small animal, such as a snake or a bird, is caught on power transmission lines or when trees make contact with power transmission lines. In this case, an “instantaneous drop” of a relatively long time, exceeding several minutes, may occur. However, providing a large converter having rated power several times the power supplied in normal time to take countermeasures for an instantaneous drop or an instantaneous power failure that does not occur frequently as described above causes problems; that is, the stable power supply apparatus is made large in size and heavy in weight, the power loss thereof increases, the cost of the equipment rises, and the expenses during operation also rise.